The present invention relates to a coating method for a building element or block of the type which has molded cavities or discontinuities in the facing surface of the block to increase the sound absorption capability of the block, commonly known as acoustical concrete block.
Coating compositions such as are disclosed in U.S. Pat. Nos. 1,761,775, 3,328,231, 3,632,725 and 4,030,289, the disclosures of which are hereby incorporated by reference and relied upon, have been successfully applied to the porous surfaces of building blocks, including cinder or concrete blocks, to impart a decorative and protective surface finish in the form of a hardened glaze to the blocks. However, it is desirous to also glaze the surface of acoustical concrete block which contain one or more sound absorbing molded cavities or discontinuities on the decorative surface. It has been found that using conventional molding systems, such as disclosed in the aforementioned patents, causes defects to occur in a large percentage of the cast decorative surfaces produced. These defects include hairline cracks in the glazed surface caused by unbalanced force vectors radiating at and surrounding the sound absorbing void or cavity on the decorative surface of the building unit. This problem occurs during the polymerization-shrinkage stage. In some circumstances, where the discontinuity is initially covered over by the coating composition and subsequently removed by cutting or the brute force of a tool, hairline cracks formed during polymerization cannot be removed and migrate into the remainder of the decorative surface. Also, with the use of brute force, additional cracking of the glaze surface has often resulted which, again, detracts from the appearance of the block as well as the performance and long term durability of the surface coating.
It is, accordingly, an object of the present invention to provide a molding method and a mold apparatus which overcomes the foregoing deficiency.
In a preferred embodiment, according to the method of the present invention, an insert member having a boundary edge closely conforming to the shape of the discontinuity in the surface of the block on which the coating composition is to be molded is provided and is employed during the curing of the resin. The insert member is inserted through the rear of the discontinuity or slot in the surface of the block and is impressed into the molding composition either while the molding composition and block are located in a molding pan such as one of those disclosed in the aforementioned U.S. patents or by pre-installing the insert in the raw block so that the block and insert are placed over the resin simultaneously. In the latter case, it is necessary to reach into the block cavity and impress the insert downward to the surface of the mold. The use of the insert to divide and separate the molding composition around the periphery of the sound absorbing voids in the face of the mold is an essential key to satisfactorily glazing an acoustical concrete block. The mechanical division of the molding compound into two surfaces at the circumference of the void prevents cracks from migrating from the unsupported compound in the void area into the supported compound in the remainder of the face of the block. This is a process of isolating the thermodynamic properties that would otherwise cause cracking. After the insert has been placed in the void so that it penetrates the molding composition, the article is then cured, and subsequent to curing, the glazed acoustical block is removed from the mold pan. Thereafter, a worker removes the insert by imparting a force to the compound in the void area of the face of the glazed acoustical block such as to drive the insert and the compound isolated by the insert down into the concrete cavity below the face of the building block. With this method and by the use of the insert, a uniform boundary or edge is provided in the glazed coating surrounding the discontinuity or opening formed in the surface of the building block.
A full appreciation of the present invention will be obtained as consideration is given to the following detailed description when taken in conjunction with the accompanying drawings, in which: